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Common specification concepts

Status: Stable, Feature-freeze

Table of Contents

Attributes

Attributes are a list of zero or more key-value pairs. An Attribute MUST have the following properties:

  • The attribute key, which MUST be a non-null and non-empty string.
  • The attribute value, which is either:
    • A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
    • An array of primitive type values. The array MUST be homogeneous, i.e. it MUST NOT contain values of different types. For protocols that do not natively support array values such values SHOULD be represented as JSON strings.

Attribute values expressing a numerical value of zero, an empty string, or an empty array are considered meaningful and MUST be stored and passed on to processors / exporters.

Attribute values of null are not valid and attempting to set a null value is undefined behavior.

null values SHOULD NOT be allowed in arrays. However, if it is impossible to make sure that no null values are accepted (e.g. in languages that do not have appropriate compile-time type checking), null values within arrays MUST be preserved as-is (i.e., passed on to span processors / exporters as null). If exporters do not support exporting null values, they MAY replace those values by 0, false, or empty strings. This is required for map/dictionary structures represented as two arrays with indices that are kept in sync (e.g., two attributes header_keys and header_values, both containing an array of strings to represent a mapping header_keys[i] -> header_values[i]).

See Attribute and Label Naming for naming guidelines.